Because of the known deleterious effects on the skin of the anionic sulfate and sulfonate detergents commonly used in shampoos, research efforts continue to be directed towards the formulation of milder shampoos which are free of such sulfated and sulfonated surfactants but which exhibit the cleansing and foaming characteristics of shampoos containing said commonly used sulfated and sulfonated surfactants. Furthermore, with the emphasis on use of naturally derived ingredients, shampoos containing water-soluble soap as a surfactant in place of said sulfated and sulfonated detergents have present-day marketing appeal, too.
Prior to the commercialization of the anionic sulfated and sulfonated detergents, e.g., sodium or triethanolamine lauryl sulfate, ammonium C.sub.8 -C.sub.18 alkyl monoglyceride sulfate, sodium C.sub.9 -C.sub.15 alkylbenzene sulfonate, etc., liquid shampoo compositions employed potassium soaps as the primary surfactant. For example, liquid shampoos containing potassium soaps are shown in Chapter 19 of "Cosmetics, Science and Technology" edited by Sagarin and in U.S. Pat. No. 4,312,771 in combination with a C.sub.8 -C.sub.18 fatty acid monoethanolamide. While such shampoos were very satisfactory in soft water, they were less than completely satisfactory in hard water due to the formation of water-insoluble calcium and magnesium soap curds.
With the commercialization of synthetic detergents such as the alkyl sulfate salts, alkylbenzene sulfonate salts, zwitterionic detergents, etc., mixtures of soap and a synthetic organic detergent often were employed in shampoos. For example, U.S. Pat. No. 3,660,470 discloses shampoo compositions containing a mixture of soap and a zwitterionic detergent in which the ratio of soap to zwitterionic detergent is from 3:1 to 100:1. Similarly, U.S. Pat. No. 3,957,970 discloses shampoos containing 50-60% by weight of soap, fatty acid alkanolamide, and 40-60% by weight of a synthetic organic detergent such as sodium lauryl sulfate or a mixture of triethanolamine lauryl sulfate and C.sub.8 -C.sub.18 alkanamidopropyl dimethyl betaine. U.S. Pat. No. 3,988,438 discloses a shampoo containing a mixture of a major proportion of soap and up to 5% by weight of the triethanolamine salt of alginic acid. On the other hand, Norda Briefs disclose soap-free shampoos containing a mixture of alkyl sulfate, zwitterionic betaine and fatty acid diethanolamide as does U.S. Pat. No. 3,950,417 which discloses shampoos containing zwitterionic betaine, alkyl sulfate and polyoxyethylene sorbitan monolaurate. However, with the exception of U.S. Pat. Nos. 3,660,470 and 3,988,488, all of the foregoing shampoo compositions contain a skin-irritating anionic sulfated or sulfonated surfactant as an essential ingredient.
Although the patents discussed above appear to suggest either mixtures of soap and anionic sulfonated or sulfated detergents or, alternatively, non-soap, anionic sulfated or sulfonated detergents for use in shampoos, some shampoos comprising mixtures of soap and zwitterionic detergents are disclosed. One such patent is U.S. Pat. No. 3,660,470 and another patent is U.S. Pat. No. 3,767,584. In these patents, soap is employed as the principal surfactant, an approach which is consistent with the teachings in U.S. Pat. No. 3,536,628 wherein a minor proportion of a zwitterionic detergent is included in a soap bar.
The inventive shampoo differs from the prior art compositions in that it is based upon a mixture of soap and zwitterionic detergent wherein the zwitterionic detergent is present in a major proportion, but in a prescribed molar ratio. Further, no anionic sulfate or sulfonated detergent is included; but, surprisingly, the resultant shampoo has cleaning and foaming properties which are equivalent to non-soap shampoos containing a mixture of alkyl sulfate and fatty alkanolamide. An additional characteristic of the inventive shampoos is their mildness.